New types of imaging-printers will depend on the extensive use of special, photographic media. Such media typically includes a carrier medium (plastic or paper-based) which is coated on one or two sides with one or more layers of special material. The resulting blank (ink receptacle) medium provides excellent image quality, but is very susceptible to media damage such as scratches, chemical surface change through contact with other materials such as surfactants, and the like. Moreover, the special photographic media usually curl extensively due to their layered/sandwich-type construction, and are fairly stiff as compared to plain paper, and therefore difficult to bend. This invention addresses several problems associated with handling such special media.
One problem is that of minimizing the necessary operating space for paper-handling and minimizing sheet deformation/sheet curl. A straight paper path requires a large printer-footprint, especially in operational conditions, in which the printer requires a pull-out tray. A large foot print is especially undesirable in a home environment. The occupied space can be reduced, if a curved paper path is applied. However, conventional curved paper paths bend the paper (sheets) through a small bending radius, which results in sheet-curl. A curved paper path in accordance with one aspect of this invention utilizes a relatively large bending radius, minimizing sheet curl, while minimizing the space required to operate the sheet-feeding device, since the sheet is wrapped around the print mechanism in order to minimize space utilization.
Another problem is a tendency of special media to jam in the paper path, and skewing of the media position while passing through the paper path. Some known curved paper paths are based on a long travel distance of the media sheets and therefore many parts inside of the paper path have to be optimized to avoid paper jams and to produce low failure rates. As a result, these paper paths are expensive to design and to manufacture, and need high maintenance as well. The paper travel distance in the paper path in accordance with an aspect of this invention is relatively short as compared with some laser printers and copy machines, which reduces the need for extensive optimization. Further, after the sheet is picked through the pick device in accordance with this aspect of the invention, it is pulled and pushed by only one roller before it is fed through the final sheet-advance roller. Sheet skew, a rotation around the sheet surface normal axis, is minimized, because the sheet never leaves the roller before it is finally fed into the sheet-advance roller.
One common type of pick system is a stationary roller pick system, where the pick roller is fixed with respect to the media path; i.e. a stationary roller system would allow only one radius of curvature for the media while picked. A stationary roller pick system is less suited for picking different media sets, such as an application which is called upon to handle standard paper as well as special photographic media. The stationary pick system is not flexible enough to adjust "itself" for the elastic properties of different media. Therefore high pick failure rates frequently occur. In addition, a stationary roller system requires more space than a "dynamic" pick system in a curved paper path in accordance with a further aspect of this invention, which pushes the sheets in an angle out of the sheet tray. On the other hand, a "dynamic" pick system allows for different radii of curvature depending on the stiffness of the picked medium and therefore allow for improved pick reliability. A dynamic pick system is used in the auxiliary sheet feeding trays of the HP1600 DeskJet and the LaserJet 4MV products marketed by the assignee of this application. The utility of this invention is independent of the pick system used, as a sheet feeding device in accordance with this invention can be employed with dynamic pick systems as well as stationary pick systems.
Another problem addressed by the invention is that of minimizing surface degradation of photographic media. Conventional curved paper paths move the paper or any sheet-media with its sensitive surface on the inside bending radius, as well as the outside bending radius, which in turn will result in a high surface degradation. In addition, sometimes the sensitive surface of the media touches corners and edges while moving through the paper path. This will result in further degradation of the print surface, especially if the surface is coated with a scratch sensitive coating found in photographic media.